The Associated Press - By By ROBERT BURNS - AP National Security Writer

December 6, 2018 9:17pm

Soldiers of the Ukrainian air assault units board a military cargo aircraft at a military base in the Zhytomyr region, Ukraine, Thursday, Dec. 6, 2018. The long-simmering conflict between Russia and Ukraine that started with Russia's annexation of Crimea spilled into the open on Nov. 25 when the Russian coast guard fired upon and seized three Ukrainian naval vessels and their crews, to which Ukrainian President Petro Poroshenko responded by introducing a martial law for 30 days and Ukraine has beefed up its forces on the border with Russia. (Mikhail Palinchak, Presidential Press Service via AP)

WASHINGTON (AP) — An unarmed U.S. Air Force plane with American and international observers flew a special flight over Ukraine Thursday in a show of solidarity after Ukraine's naval confrontation last month with Russia in the Black Sea, the Pentagon said.

In a statement, the Pentagon called it an "extraordinary" flight under the Open Skies Treaty, which is an international accord that gives each member country the right to conduct, and the obligation to accept, unarmed military observation flights. The purpose to is promote transparency in military activities.

Normally these flights are arranged well in advance. But the treaty also permits "extraordinary," or extra, flights if two participating members agree. In this case, the Ukrainian military requested a flight and the U.S. agreed to conduct it over an uncontested portion of Ukraine territory, according to Eric Pahon, a Pentagon spokesman.

Pahon said this was the first "extraordinary" Open Skies flight since 2014 in response to Russia's annexation of Crimea.

The Navy, meanwhile, is expected to sail a warship into the Black Sea later this month, a passage that American ships have done a number of times this year. The transit would be another signal of military support for Ukraine.

The U.S. has notified Turkey of the plan, which is part of the required process, U.S. officials said. It kicks off a 15-day wait before the ship can move into the Black Sea. The officials spoke on condition of anonymity to discuss future ship movements.

U.S. Navy ships routinely sail in and out of the Black Sea, and under international rules are only allowed to stay in the waters for 21 days. The most recent was the fast transport ship USNS Carson City, in September. Officials wouldn't say which ship will make the passage in mid-December.

On Nov. 25, Russia and Ukraine had a naval clash near Kerch Strait. Since then, Washington has sought to show its support for Ukraine while condemning Russian actions.

"The timing of this flight is intended to reaffirm U.S. commitment to Ukraine and other partner nations," the Pentagon said. "Russia's unprovoked attack on Ukrainian naval vessels in the Black Sea near the Kerch Strait is a dangerous escalation in a pattern of increasingly provocative and threatening activity," it added. "The United States seeks a better relationship with Russia, but this cannot happen while its unlawful and destabilizing actions continue in Ukraine and elsewhere."

Pahon said the plane that was flown over Ukraine on Thursday was an OC-135 observation aircraft with U.S., Canadian, German, French, British, Romanian and Ukrainian observers aboard. He said the flight was requested by the arms control directorate of the Ukrainian General Staff.

Pahon said the plane had a 17-member crew from the 55th Wing based at Offutt Air Force Base in Nebraska. It flew from Andrews Air Force Base in Maryland to Ramstein air base in Germany on Nov. 30 and flew over Ukraine on Thursday, he said.